The invention relates to photocatalytic processes utilizing molecular sieves such as zeolites. Specifically, this invention relates to photocatalysts that include optical fibers coated with particles of molecular sieves, and methods for their use in photocatalytic processes.
The use of molecular sieves, especially zeolites, in photochemistry has attracted significant interest as they host a variety of organic transformations in their cavities and channels that often leads to product distributions considerably different from those in solution (1, 2, 3, 4). Such studies have shown that zeolites provide a microenvironment for these reactions and retard dramatically back-transfer of electrons that enormously increases the lifetime of the photo-generated ion pairs. Recently, the group of Frei et al. used large-pore alkali or alkaline-earth zeolites (type Y or L), molecular oxygen, and visible light to achieve partial oxidation of small alkanes, olefins and alkylbenzenes with unprecedented selectivity (5, 6). These reactions were conducted at ambient temperature and in the absence of solvent or photosensitizers.
Oxidation reactions are one of the most important chemical processes in industry. Some of the partially oxygenated hydrocarbons find application in the manufacture of plastics and synthetic fibers and as intermediates in the synthesis of fine chemicals (7, 8) Moreover, this concept can be used to convert organic pollutants or unwanted side products such as partially hydrogenated compounds into more useful products. In order to scale-up the micromolar-quantity experiments, it is essential to improve upon the scattering of visible light by the zeolite matrix, the ability of reactants and products to diffuse through the catalyst bed, and the development of a continuous flow reaction system (6).
Although numerous literature is available on the deposition of zeolite crystallites on various substrates from a hydrothermal reaction gel, there are very few examples of three-dimensional objects that have been coated with films of pre-synthesized zeolites (9, 10). Moreover, coatings prepared by direct crystal deposition from solution often suffer from defects and poor adhesion. Therefore, there is a need for a molecular sieve-coated substrate system, such as a zeolite coated substrate system, which can decrease light scattering, improve the ease of diffusion of reactants and products, and is robust enough to tolerate the operation and severe conditions of typical commercial flow reactors. The present invention addresses this need.
It has been discovered that photocatalysts having improved photoefficiency can be synthesized by coating molecular sieves on quartz optical microfibers. Accordingly, one aspect of the invention provides a photochemical catalyst that includes an optical microfiber coated with a catalytic amount of a molecular sieve, the optical microfiber being effective to transmit light to the molecular sieve. In preferred forms of the invention, a photochemical catalyst includes an optical microfiber having at least a portion of the microfiber free of cladding, wherein a particulate molecular sieve is coated upon the cladding-free portion.
In another aspect of the invention, methods of making a photochemical catalyst are provided. In one form of the invention, a method includes bonding or otherwise depositing a catalytic amount of a particulate molecular sieve onto an outer surface of an optical microfiber, wherein the microfiber is effective to transmit light to the bonded molecular sieve. For example, at least a portion of the outer surface of the microfiber can be cladding-free, with the particulate molecular sieve bonded to the cladding-free portion.
Although a wide variety of molecular sieves are envisioned, in one preferred form of the invention, the molecular sieve is a zeolite.
In yet other aspects of the invention, the molecular sieve of the photocatalyst may include dye sensitizer molecules or other light absorbing or directing components or molecules incorporated in or otherwise anchored to the molecular sieve, preferably inside the cavities of the sieve. Such sieves form another inventive photocatalyst when anchored on an optical microfiber. Such a photocatalyst may be advantageously used to perform, for example, photosensitizer-induced electron transfer reactions. In a further form of the invention, the light absorbing or directing component may include semiconductor particles, such as TiO2 or SnO2, that are incorporated into the molecular sieve, or otherwise anchored inside the cavities of the molecular sieve. Alternatively, transition metal ions, such as Ti4+, W5+, or V5+, may be incorporated into the framework of the molecular sieve, or grafted onto its interior surfaces.